1. Field of the Invention
The present invention generally relates to a sound field processor, and more particularly, to a sound field expanding apparatus that expands the sound field of a two-channel stereophonic sound and that is readily implemented in LSIs.
2. Description of Related Art
In a conventional two-channel stereophonic apparatus, sound distribution that is generated by right and left speakers of the apparatus is limited within a range between the right loudspeaker and the left loudspeaker. A sound field expanding apparatus is used to expand the sound distribution to areas outside the speakers. In expanding the sound field of a two-channel stereophonic sound, the amplitude characteristic and/or the phase characteristic of a signal in each of the two channels is altered or changed. Then, the two signals are added to each other.
For example, when a specified amplitude-phase characteristic is added to an L-channel sound signal, an inverse characteristic of the specified amplitude-phase characteristic is added to the L-channel sound signal. Then the L-channel sound signal is mixed with an R-channel sound signal. In order to correctly provide the inverse characteristic, a constant of a circuit for changing the amplitude/phase characteristic and a constant of a circuit for changing the inverse characteristic must be set precisely. However, it is very difficult to set precisely such constants in the circuits due to qualitative variations in devices used in the circuits.
To address these problems, the applicant has proposed a sound field expanding apparatus, shown in FIG. 5 (see Japanese patent publication HEI 3-80400). The sound field expanding apparatus is formed from an L-channel sound field expanding circuit A and an R-channel sound field expanding circuit B that have substantially the same structure. An amplitude/phase characteristic changing circuit 37 (37a, 37b) in each of the circuits A and B changes an amplitude/phase characteristic "X (f) e.sup.j .theta.(f)" of an output signal from an inversion amplifier 36 (where X is an amplitude gain, and .theta. is a phase variation amount). Then, a signal from the amplitude/phase characteristic changing circuit 37 is outputted to an adder of the opposite channel. More particularly, a signal from the amplitude/phase characteristic changing circuit 37a is outputted to an adder 48 of the R-channel sound field expanding circuit B, and a signal from the amplitude/phase characteristic changing circuit 37b is outputted to an adder 46 of the L-channel sound field expanding circuit A. Also, the inversion amplifier 36 has a feedback loop that is defined by an output terminal of the inversion amplifier 36.fwdarw., the amplitude/phase characteristic changing circuit 37.fwdarw., an adder 44.fwdarw., an adder 34.fwdarw., and an inversion input terminal of the inversion amplifier 36. An output signal V2 from the inversion amplifier 36 is represented by the following formula: EQU V2=-V1/{1+X(f)e.sup.j .theta.(f)}
When signals L and R are inputted as input signals Ain and Bin to input terminals 33a and 33b, respectively, the following output signals Aout and Bout are output from output terminals 50 and 52, respectively: EQU Aout={L+R.multidot.X(f)e.sup.j .theta.(f)}/{1+X(f)e.sup.j .theta.(f)} EQU Bout={R+L.multidot.X(f)e.sup.j.theta.(f)}/{1+X(f)e.sup.j .theta.(f)}
Also, the amplitude/phase characteristic changing circuit 37 is formed from, for example, a secondary low-pass active filter shown in FIG. 6. The secondary low-pass active filter shown in FIG. 6 includes resistors having resistance values R1, R2 and R3, and capacitors having capacity values Cx and Cy. In this case, a cut-off angular frequency W is set to an aural signal band in order to obtain an effect of expanding the sound field, and the cut-off angular frequency W is represented by the following formula: EQU W=1/(R2.multidot.R3.multidot.Cx.multidot.Cy).sup. 1/2
It is desirable to implement the above-described sound field expanding apparatus in an LSI in order to reduce the number of devices and parts and to improve the reliability of the sound field expanding apparatus. In such a case, as described above, the cut-off angular frequency W of the amplitude/phase characteristic changing circuit 37 is set to the aural signal band. As a result, the resistance value and the capacity value increase substantially. For example, the capacitance value of the capacitors amounts to several hundreds pF to several thousands pF. Accordingly, when a sound field expanding apparatus is implemented in an LSI, the area required for the capacitors on the IC chip becomes substantially large and therefore the cost of the LSI increases.
Also, when different effects in expanding the sound field are desired to be switched, plural resistors and capacitors for the amplitude/phase characteristic changing circuit 37 have to be included in advance. When a user wants to add a specified characteristic to change a given sound field effect, these multiple resistors and capacitors have to be switched in accordance with the specified characteristic that is desired. In this case, more capacitors are required, and implementation of the apparatus in an LSI becomes difficult.